Glaucoma is the name given to a group of conditions in which the optic nerve suffers a characteristic form of damage at the back of the eye which is often associated with a raised level of intraocular pressure.
The optic nerve damage causes patchy loss of vision that varies in severity from patient to patient.
Without treatment, the loss of vision usually gets worse over the course of many months or several years. The loss of vision in glaucoma is permanent, but with early treatment, the damage to vision can be minimised.
Most patients with glaucoma are not aware of problems with their vision. This is because the central vision (for reading and recognising people) is only affected when glaucoma has advanced to a late stage.
Even when central vision is still good, glaucoma may affect the vision needed for driving and getting about (for instance, seeing steps). Blindness from glaucoma is rare. If blindness does occur, it is usually because the glaucoma is already advanced when it is first diagnosed, because the eye pressure has not responded well to treatment or because patients have not taken their drops regularly.
For most glaucoma patients, the main effect of the condition is the inconvenience of taking eye drops.
What are the different types of glaucoma?
Adult glaucoma falls into two categories – open angle glaucoma and closed angle glaucoma. These categories are subdivided, according to whether the cause is unknown (primary glaucoma) or known i.e. the high eye pressure is caused by other conditions of the eye (secondary glaucoma).
A certain level of pressure is needed within the eye for it to keep its shape. This pressure is maintained by the flow of a fluid (aqueous humour) within the eye, and should not be confused with tears.
A balance between the fluid entering and leaving the eye determines the pressure in the eye (the intraocular pressure).
Most cases of glaucoma occur because the flow of fluid out of the eye becomes restricted and the pressure within the eye rises. This pressure causes damage to the optic nerve.
In some eyes with glaucoma, the pressure is not high. In these cases it is thought that a poor blood supply or a weakness in the optic nerve structure may make the nerve susceptible to damage by normal eye pressure. This is known as normal tension glaucoma.
Some eyes can have moderately raised pressure but no signs of loss of vision or optic nerve damage and this is known as ocular hypertension.
Are some people at increased risk of developing glaucoma?
Yes, there are several risk factors which make the onset of glaucoma more likely and they tend to be cumulative in their effect.
Primary open angle glaucoma (POAG) becomes much more common with increasing age. It is uncommon below the age of 40, but the number of people with the condition rises from about 2 per cent of people over the age of 40 and doubles for those over the age of 80.
People of African-Caribbean origin have about a four times increased risk of POAG when compared with those of a European origin. The condition also tends to come on at an earlier age and be more severe. Regular testing is therefore vital if visual impairment is to be avoided. People of Asian origin are at an increased risk of developing primary angle closure glaucoma (PACG).
There is at least a four times increased risk of developing glaucoma if you have a close blood relative with the condition (father, mother, brother, sister or child) which is why early detection is so important as treatment is more effective when the glaucoma is detected early. Eye examinations are funded by the NHS for such people from the age of 40 years, but an earlier test is recommended, especially if you also fall into one of the other risk categories. Regular eye health checks should include the three glaucoma tests: ophthalmoscopy, tonometry and perimetry.
If you have glaucoma, don’t forget to tell your relatives about the condition and the need for them to be tested. More information can be found in the IGA leaflet entitled 'Glaucoma and your relatives'.
People with severe myopia (very short sight) are known to be at increased risk of developing glaucoma, and should ensure that they are regularly tested for glaucoma.
Long sighted people are known to be at increased risk of developing angle closure.
People with diabetes may be at increased risk of developing glaucoma, although it is not known whether there is a direct link between the two conditions. However, all people with diabetes should have regular routine eye examinations for diabetic eye diseases and glaucoma tests can usually be requested at the same time.
Everyone over 40 years of age should ensure that they have regular sight tests every two years to discover if their eyesight has deteriorated. These tests are a good opportunity to check for any signs of glaucoma. During these visits, make sure you ask the optometrist (optician) to carry out the three glaucoma tests:
1. Eye pressure (tonometry)
Tonometry is the measurement of intraocular pressure (the pressure inside the eye) and can be carried out with several different instruments. In general, Goldmann applanation tonometry (with the blue light) is used in hospitals and non-contact (air puff) tonometry is used in optometric (optician) practices. Goldmann applanation tonometry involves local anaesthetic drops (with a yellow dye called fluorescein) being given to allow the instrument to touch the front of the eye. The drops sting a little, but the procedure itself is completely painless. It is generally considered to be the most accurate method of measuring the pressure.
In non-contact tonometry, a puff of air is blown at the eye from the instrument. This does not require any form of anaesthetic and is generally considered to be somewhat less accurate. All devices that measure the eye pressure flatten the cornea to some extent. Eyes which have a thicker (or stiffer) than average cornea may give rise to an overestimate of the eye pressure. Similarly, eyes with a thin (or soft) cornea may give rise to an under-estimate of the eye pressure. For this reason, when the eye pressure is measured in hospitals, a measurement of the thickness of the cornea may also be taken. Alternatively, the pressure may be taken with a new device (such as the Pascal Dynamic Contour Tonometer or the Ocular Response Analyzer) that is less affected by the properties of the cornea.
2. Optic disc appearance (ophthalmoscopy)
The appearance of the optic disc can be examined using an ophthalmoscope (a special sort of torch) or by the use of a slit lamp. This allows the examiner to assess the degree of cupping of the optic disc and the health of the retina.
3. Visual field (perimetry)
Perimetry is the technique of mapping out the blank or less sensitive areas in the field of vision, so that the presence of optic nerve damage can be assessed. When these three tests are used together, the likelihood of detecting glaucoma increases by fourfold when compared with ophthalmoscopy alone.
The three tests are quick and painless and, depending on the instruments used, should only take around 15 minutes in addition to the normal eye examination.
Glaucoma of some type is found in about 2 per cent of the population over the age of 40. It can also affect children and young adults, although much less frequently.
It is estimated that more than 500,000 people suffer from glaucoma in England and Wales alone, with more than 70 million people affected across the world.
The great majority of those with glaucoma have a chronic (slowly developing) form of the condition, primary open angle glaucoma (POAG), and studies have demonstrated that half of all cases remain undiagnosed. In other words, 250,000 people in England and Wales are still unaware that they are slowly losing their sight.
The treatment of glaucoma has developed considerably over recent years and new, more potent drugs with fewer side effects than earlier medications are now available.
Surgical techniques have also improved and it is estimated that around 95 per cent of those diagnosed early with glaucoma in the UK will retain useful sight for life.
Although blindness from glaucoma is uncommon, it is responsible wholly or in part for 13 per cent of those on the blind register in England and Wales and is one of the leading causes of preventable blindness in the UK .